<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        var flag=true;
        console.log(flag+1);  //2

        var str;
        console.log(str);
        console.log(str+1);    //NaN
        console.log(str+'鲁大师'); //undefined鲁大师

        var str2=null;
        console.log(str2)
        console.log(str2+1);   //1   null和数字相加还是数字
        console.log(str2+'pink');   //nullpink

    </script>
</head>
<body>

</body>
</html>